The present invention relates to electrochemical acidification of aqueous solutions containing alkali metal sulfate and chlorate. The inventions also relates to production of chlorine dioxide comprising electrochemical acidification of effluents and recirculation of the acidified effluents to the chlorine dioxide reactor.
Electrochemical acidification of aqueous solutions containing sulfate and/or chlorate is of considerable interest, particularly in connection with recovery of by-products from process of producing chlorine dioxide.
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fan bleaching, removal of phenols from industrial wastes etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
There are numerous different processes for chlorine dioxide production. Most processes in commercial use involve reaction of sodium chlorate in an acidic medium with a reducing agent such as hydrogen peroxide, methanol, chloride ions or sulfur dioxide. The acidity is generally provided by sulfuric acid. A drawback of the known processes is the formation of some form of sodium sulfate as a by-product which has to be removed from the reactor, either in the form of a solid saltcake or as waste acid. Most modern processes are operated under subatmospheric pressure, involving precipitation of the sodium sulfate as a saltcake which has to be removed from the reactor by filtering. Today it is hard to find any use for the salt cake and it is normally regarded as an unwanted by-product.
In order to avoid formation of a sulfate by-product, it has been disclosed to provide all acid needed for the chlorine dioxide generation from chloric acid which can be prepared electrochemically from sodium chlorate. Such methods are described in, for example, the U.S. Pat. Nos. 4,915,927, 5,084,148 and 5,174,868. However, it has been found difficult to achieve satisfactory current efficiency in production of strong chloric acid which is desirable in order provide efficient chlorine dioxide generation.
International patent specification WO 93/25470 discloses a process of producing chlorine dioxide from chloric acid and hydrogen peroxide. The chloric acid can be produced electrochemically.
U.S. Pat. No. 4,806,215 discloses a process in which chlorine dioxide is generated from sodium chlorate and hydrochloric acid, in which process the generator liquor is acidified electrochemically and recycled back to the reactor. However, this process necessarily results in co-formation of chlorine which cannot be accepted in modern environmental friendly bleaching processes.
U.S. Pat. No. 4,129,484 discloses a process of producing chlorine dioxide in which process sulfuric acid and sodium hydrogen sulfate is withdrawn from the reactor and subjected to electrolysis. However, the current efficiency obtained in the electrochemical cell is not satisfactory.
U.S. Pat. Nos. 5,198,080 and 5,122,140 disclose a process of producing chlorine dioxide involving crystallisation and withdrawal of solid sodium sesquisulfate. The solid salt is dissolved again, electrolytically acidified and recycled to the chlorine dioxide reactor. High current efficiency is said to be achieved if sodium chlorate is added to the dissolved sulfate before the electrochemical cell.
International patent specification WO 93/19227 discloses that electrochemical acidification of alkali metal sulfate solutions preferably is performed at a water content below about 55% by weight.
It is an object of the invention to provide an electrochemical process for acidification of an aqueous solution containing sulfate and chlorate which can be operated efficiently for a long time. It is another object of the invention to provide an efficient process of producing chlorine dioxide involving electrochemical acidification of by-products obtained.